The study of extreme environments is an exploration of the limits of life. Organisms perform a number of basic functions (homeostasis, metabolism, growth, reproduction, etc.), and our water- and carbon-based systems are constrained within certain environmental parameters. Some organisms can push the limits of these environmental boundaries and thrive in what to most other living things are conditions inimical to life. Thus the concept of “extreme” environment is necessarily relative to conditions under which most species thrive. Organisms that live in relatively hostile environments (called extremophiles) include archaea and bacteria, but other groups of organisms also have members that can live in relatively stressful habitats. Scientists point out that there is a difference between living under extreme conditions and tolerating (perhaps by going dormant) extreme conditions, but both situations can help us understand how extreme environments affect life. The adaptations that allow organisms to live in (or survive) extreme conditions are targets of scientific study because they help us understand life’s basic processes and how life responds to environmental challenges. The lessons we learn have important applied aspects because they can help us grow food, process wastes, restore disturbed habitats, and perform many other vital tasks. In this article, we provide sections based on particularly important stress factors, but we also have included sections in which the focus is on major concepts, to show how organisms from extreme environments can inform other areas of scientific interest.